170 Dr. J. C. Bose. On the Determination of the 



where a + b is the sum of breadths of strip and space in the grating, 

 i = angle of incidence, 6 = angle of diffraction. The sign of 9 is 

 taken positive when it lies on the same side of the normal as the 

 incident radiation. 



In the above equation there are two interesting cases : 



(1) When the receiver is placed at C, = 



(a + 6) sini = n\. 



(2) When the deviation is minimum i = 



2 (a + 6) sin i = n\. 



Arrangement of the Apparatus. 



The grating, which is cylindrical, is placed vertically on a wooden 

 table, with its centre at C, occupied in the diagram by the spiral 

 spring coherer S. With the radius, which joins the centre to the 

 apex of the grating, as a diameter, a circle is engraved on the table 

 the focal curve on which the radiator and the receiver are always 

 kept. A pin is fixed immediately below the apex, and a graduated 

 ring sunk in the table with this pin as the centre. The graduated 



FIG. 2. 



The radiator, R, and the receiver, S, revolve round a pivot vertically below the 

 apex of the grating, along the focal curve. The angles aro measured by the 

 graduated circle, D. 



circle is used for the measurement of the angles of incidence and 

 diffraction. Two radial arms revolving round the pin carry the 

 radiator and the receiver. The ends of the arms near the pin have 



